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In order to enhance the CO 2 dissolution rate and prolong gas− liquid contact time to improve microalgal growth, a spiral-ascending CO 2 dissolver was developed to enhance CO 2 mass transfer in a horizontal tubular photobioreactor. A spiral-ascending flow pattern of CO 2 bubbles was formed with a helical baffle and central hollow tube in the CO 2 dissolver, which markedly extended flow trajectory of CO 2 bubbles and intensified the mixing effect. This novel CO 2 dissolver with the helical baffle shortened the bubble generation time by 30.6% and decreased the bubble generation diameter by 23.4%, thereby enhancing the mass transfer coefficient by 69.2%. The bubble retention time dramatically increased by 190.2%, which prolonged CO 2 gas−liquid contact time to enhance dissolved CO 2 concentration. The liquid phase mixing time decreased by 15.8% to give a uniform status in microalgal suspension, allowing for sufficient nutrient supply for photosynthesis during microalgal circulation in horizontal tubes. This led to an increased microalgal biomass accumulation by 40.8% with 15% CO 2 in the horizontal tubular photobioreactor.

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